For the past several weeks, we have been keeping an eye out for Microsoft's viral marketing campaign for its Surface tablet. The campaign consists of ads painted on the sides of buildings, graffiti style. So far, the cities of New York, Los Angeles and Chicago have had select locations picked for these ads.

Now the campaign has expanded with the first confirmed appearance of a Surface ad on the streets of San Fransisco. Twitter user Noel Adolph posted up a photo of painters creating the latest known Surface street art ad.

In a later post, Adolph said he was simply walking past the ad, which was being created on the corner of Filmore Street and Sutter Street. It's likely that this will be just one of many such street art ads for the tablet in San Francisco.

With the launch of the Windows RT version of Surface now less than four weeks away, we can expect to see even more of these ads, with the "Click in" slogan, pop up in a number of other major US cities. Microsoft has still not commented officially on this viral campaign, most likely in an effort to keep some sense of mystery on where the next ads might show up.